The present invention relates to a process for the recovery of olefins from cracked gases employing a chemical absorption process.
The processes for converting hydrocarbons at high temperature, such as for example, steam-cracking, catalytic cracking or deep catalytic cracking to produce relatively high yields of unsaturated hydrocarbons, such as, for example, ethylene, propylene, and the butenes are well known in the art. See, for example, Hallee et al., U.S. Pat. No. 3,407,789; Woebcke, U.S. Pat. No. 3,820,955, DiNicolantonio, U.S. Pat. No. 4,499,055; Gartside et al., U.S. Pat. No. 4,814,067; Cormier, Jr. et al., U.S. Pat. No. 4,828,679; Rabo et al., U.S. Pat. No. 3,647,682; Rosinski et al., U.S. Pat. No. 3,758,403; Gartside et al., U.S. Pat. No. 4,814,067; Li et al., U.S. Pat. No. 4,980,053; and Yongqing et al., U.S. Pat. No. 5,326,465.
It is also well known in the art that these mono-olefinic compounds are extremely useful in the formation of a wide variety of petrochemicals. For example, these compounds can be used in the formation of polyethylene, polypropylenes, polyisobutylene and other polymers, alcohols, vinyl chloride monomer, acrylonitrile, methyl tertiary butyl ether and other petrochemicals, and a variety of rubbers such as butyl rubber.
Because the mono-olefins contained in the cracked gases typically contain a large amount of other components, such as diolefins, acetylenes, hydrogen, carbon monoxide and paraffins, it is highly desirable to separate the mono-olefins into relatively high purity streams of the individual mono-olefinic components. To this end a number of processes have been developed to make the necessary separations to achieve the high purity mono-olefinic components.
Multi-stage rectification and cryogenic chilling trains have been disclosed in many publications. See, for example Perry""s Chemical Engineering Handbook (5th Edition) and other treatises on distillation techniques. Recent commercial applications have employed technology utilizing dephlegmator-type rectification units in chilling trains and a reflux condenser means in demethanization of gas mixtures. Typical rectification units are described in Roberts, U.S. Pat. No. 2,582,068; Rowles et al., U.S. Pat. No. 4,002,042, Rowles et al., U.S. Pat. No. 4,270,940, Rowles et al., U.S. Pat. No. 4,519,825; Rowles et al., U.S. Pat. No. 4,732,598; and Gazzi, U.S. Pat. No. 4,657,571. Especially successful cryogenic operations are disclosed in McCue, Jr. et al., U.S. Pat. No. 4,900,347; McCue, Jr., U.S. Pat. No. 5,035,732; and McCue et al., U.S. Pat. No. 5,414,170.
In a typical conventional cryogenic separation process, as shown in FIG. 1, the cracked gas in a line 2 is compressed in a compressor 4. The compressed gas in a line 6 is then caustic washed in a washer 8 and fed via a line 10 to a dryer 12. The dried gas in a line 14 is then fed to the chilling train 16. Hydrogen and methane are separated from the cracked gas by partially liquefying the methane and liquefying the heavier components in the chilling train 16. Hydrogen is removed from the chilling train 16 in a line 18 and methane is removed via a line 20, compressed in a compressor 24 and recovered in a line 26.
The liquids from the chilling train 16 are removed via a line 22 and fed to a demethanizer tower 28. The methane is removed from the top of the demethanizer tower 28 in a line 30, expanded in a turboexpander 32 and sent to the chilling train 16 as a refrigerant via a line 34. The C2+ components are removed from the bottom of the demethanizer tower 28 in a line 36 and fed to a deethanizer tower 38. The C2 components are removed from the top of the deethanizer tower 38 in a line 40 and passed to an acetylene hydrogenation unit 42 for selective hydrogenation of acetylene. The effluent from the C2 hydrogenation unit 42 is then fed via a line 44 to a C2 splitter 46 for separation of the ethylene, removed near the top of splitter 46 in a line 48, and ethane, removed from the bottom of splitter 46 in a line 50. Lighter gases are vented from the top of the splitter 46 in a line 49.
The C3+ components removed from the bottom of the deethanizer tower 38 in a line 52 are directed to a depropanizer tower 54. The C3 components are removed from the top of the depropanizer tower 54 in a line 56 and fed to a C3 hydrogenation reactor 58 to selectively hydrogenate methyl acetylene and propadiene. The effluent from the C3 hydrogenation unit 58 in a line 60 is fed to a C3 splitter 62 wherein the propylene and propane are separated. The propylene is removed from the top of the C3 splitter 62 in a line 64 and the propane is removed from the bottom of the C3 splitter 62 in a line 66.
The C4+ components removed from the bottom of the depropanizer tower 54 in a line 68 are directed to a debutanizer 70 for separation into C4 components and C5+ gasoline. The C4 components are removed from the top of the debutanizer 70 in a line 72 and the C5+ gasoline is removed from the bottom of the debutanizer 70 in a line 74.
However, cryogenic separation systems of the prior art, such as shown in FIG. 1, while meeting with a relatively good amount of commercial success, have suffered from various drawbacks. In conventional cryogenic recovery systems, the cracked gas is typically required to be compressed to about 450-500 psig, thereby requiring 4-5 stages of compression. Additionally, in conventional cryogenic recovery systems, five towers are required to separate the C2 and C3 olefins from the paraffins: a demethanizer, a deethanizer, a C2 splitter, a depropanizer and a C3 splitter. Because the separations of ethane from ethylene and propane from propylene, involve close boiling compounds, the splitters generally require very high reflux ratios and a large number of trays, such as on the order of 120 to 250 trays each. The conventional cryogenic technology also requires multi-level propylene and ethylene refrigeration systems, as well as complicated methane turboexpanders and recompressors or a methane refrigeration system, adding to the cost and complexity of the conventional technology. Moreover, in conventional cryogenic technology the driers are required to dry the entire cracked gas stream thereby increasing their duty.
It has also been studied in the prior art to employ metallic salt solutions, such as silver and copper salt solutions, to recover olefins, but none of the studied processes have been commercialized to date. A note is made that a commercial Hoechst plant recovering olefins from cracked gases was operated at their Gendorf works in Germany during the 1950""s and early 1960""s which used cuprous nitrate and an ethanolamine ligand.
For example, early teachings regarding the use of copper salts included Uebele et al., U.S. Pat. No. 3,514,488 and Tyler et al., U.S. Pat. No. 3,776,972. Uebele et al. ""488 taught the separation of olefinic hydrocarbons such as ethylene from mixtures of other materials using absorption on and desorption from a copper complex resulting from the reaction of (1) a copper(II) salt of a weak ligand such as copper(II) fluoroborate, (2) a carboxylic acid such as acetic acid and (3) a reducing agent such as metallic copper. Tyler et al. ""972 taught the use of trialkyl phosphines to improve the stability of CuAlCl4 aromatic systems used in olefin complexing processes.
The use of silver salts was taught in Marcinkowsky et al., U.S. Pat. No. 4,174,353 wherein an aqueous silver salt stream was employed in a process for separating olefins from hydrocarbon gas streams. Likewise, Alter et al., U.S. Pat. No. 4,328,382 taught the use of a silver salt solution such as silver trifluoroacetate in an olefin absorption process.
More recently, Brown et al., U.S. Pat. No. 5,202,521 taught the selective absorption of C2-C4 alkenes from C1-C5 alkanes with a liquid extractant comprising dissolved copper(I) compounds such as Cu(I) hydrocarbonsulfonate in a one-column operation to produce an alkene-depleted overhead, an alkene-enriched side stream and an extractant rich bottoms.
Special note is also made of Davis et al., European Patent Application EP 0 699 468 which discloses a method and apparatus for the separation of an olefin from a fluid containing one or more olefins by contacting the fluid with an absorbing solution containing specified copper(I) complexes, which are formed in situ from copper(II) analogues and metallic copper.
However, none of the absorption processes to date have described a useful method of obtaining relatively high purity olefin components from olefin-containing streams such as cracked gases. The use of silver nitrate solutions, while good at separating olefins from non-olefinic hydrocarbon gases in the absence of hydrogen, cannot be used with hydrogen-containing cracked gases as the silver ions are readily reduced to metallic silver. Moreover, the use of silver nitrate solutions is significantly less attractive from an economic standpoint due to the cost of these solutions.
Regarding the copper absorption processes, none of the processes disclosed to date have proven sufficient to provide the high olefin purities for the petrochemical industry, i.e., chemical grade ethylene and propylene such as above about 95 weight percent purity; and polymer grade ethylene and propylene, such as above about 99 weight percent purity, a more preferably above about 99.9 weight percent purity.
Therefore, it would constitute a significant improvement in the state of the art if a chemical absorption process was developed to produce high purity olefins and which improved upon the conventional cryogenic systems.
It is therefore an object of the present invention to provide a process for the recovery of olefins which is sufficient to produce the olefins at high purity levels, i.e., polymer grade.
It is a further object of the present invention to provide a process for the recovery of high purity olefins which reduces the compressor requirements.
It is another object of the present invention to provide a process for the recovery of high purity olefins which eliminates the need for distillation separation of close boiling olefins and paraffins.
It is still another object of the present invention to provide a process for the recovery of high purity olefins which reduces refrigeration requirements.
It is a still further object of the present invention to provide a process for the recovery of high purity olefins which reduces drier requirements.
It is still another further object of the present invention to provide a process for the recovery of high purity olefins where the majority of the heat is supplied by low cost quench water.
To this end, the present invention provides a process for the production of high purity olefin components employing a separation system based on the separation of olefins from paraffins employing selective chemical absorption of the olefins, desorption of the olefins from the absorbent, and separation of the olefins into high purity components by distillation, thereby overcoming the shortcomings of the prior art processes.